1. Field of the Invention
The present invention relates to a laser processing, and more particularly to method and system of a laser processing, which perform a piercing process to form a through hole at a starting point of cutting in a workpiece before initiating a laser cutting.
2. Description of the Related Art
A laser cutting, known as one type of a laser processing, is generally performed by using heat of a laser beam to melt a workpiece and by blowing an assist gas to remove the molten material of the workpiece. In some cases, high-purity oxygen may be used as the assist gas so as to effectively use the heat of combustion of the workpiece in addition to the heat of the laser beam. At the time of initiating the laser cutting, it is desirable to ensure a condition where the molten material of the workpiece can be removed by the blown assist gas from a back surface of the workpiece opposite to, a front surface thereof irradiated with the laser beam. Thus, in the laser cutting, it is known that a so-called “piercing” process is performed so as to produce a through hole at a starting point of cutting in the workpiece before initiating the laser cutting.
Japanese Letters Patent No. 3110504 (JP3110504B) describes a laser processing method performing a piercing process including a first step of forming a recess on a surface of a workpiece under a laser power output and an assist gas pressure, the values of which are higher than set values required for forming a through hole, in a state where the focal point of the laser beam is spaced from the surface of the workpiece by a predetermined distance, and a second step of forming a through hole under a laser power output and an assist gas pressure, the values of which are lower than those in the first step, in a state where the focal point is shifted by a predetermined amount in a direction entering the internal part of the workpiece. It is described that, in the first step, a shallow recess with a large diameter is instantaneously formed on the surface of the workpiece, and in the second step, a through hole is successively formed from the bottom of the recess, so that it is possible to efficiently supply the assist gas into the recess and perform a good piercing in a stable manner. It is also described that, in the second step, the surface of the workpiece is irradiated with the laser beam under the laser power output and the assist gas pressure, which are set lower than those in the first step to the extent that an explosion is prevented from occurring on the surface of the workpiece, while a gap size is gradually reduced.
Japanese Unexamined Patent Publication (Kokai) No. 2007-75878 (JP2007-75878A) describes a laser processing method including a first step of irradiating a workpiece with a laser beam under a first piercing condition so as to start a piercing process, a second step of halting the irradiation with the laser beam for 0.5 seconds or more after the first step, and a third step of irradiating a point processed in the first step with a laser beam under a second piercing condition after the second step so as to complete the piercing process. It is described that, since a beam irradiation is temporarily halted in the second step after a pierced hole is partially formed to the middle of the thickness of the workpiece under the first piercing condition selected in the first step, a partially progressing oxidation/combustion reaction is interrupted, and a pierced hole with a reduced diameter can be formed by the subsequent irradiation with the laser beam in the third step since there is no excess heat input, so that the amount of spatter can be reduced and a processing time can be shortened. It is also described that a spatter anti-adhesive agent may be sprayed before the first piercing process in the first step, air and a spatter anti-adhesive agent may be sprayed in the second step, and air may be sprayed after finishing the second piercing process in the third step, so that it is possible to effectively prevent the spatter from remaining on and adhering to an area surrounding the pierced hole.
Japanese Unexamined Patent Publication (Kokai) No. 7-9175 (JP7-9175A) describes a piercing method for a laser beam machine, in which a piercing is started at a high position above a workpiece, a machining head is then lowered to a cutting height while changing machining conditions so as to prevent a molten metal from blowing up, and the piercing is further performed at the cutting height, so that it is possible to perform the piercing in a short time without generating the blowing up of the molten metal.
Japanese Unexamined Patent Publication (Kokai) No. 10-323781 (JP10-323781A) describes a laser processing method wherein a laser output is set as a continuous output when a piercing starts, and the laser output is changed to a pulse output in the course of the piercing, so as to complete the piercing.
In the laser processing method of JP3110504B, since the through hole is formed, in the second step, in the bottom of the shallow recess with the large diameter formed in the first step, under the laser power output and the assist gas pressure, which are set lower than those in the first step to the extent that an explosion is prevented from occurring on the surface of the workpiece, it is difficult to shorten the time required for the piercing process. On the other hand, the laser processing method of JP2007-75878A is described so that the effect of shortening the processing time is ensured. However, in JP2007-75878A, means for preventing the spatter from remaining on and adhering to an area surrounding the pierced hole is provided, and the second step for cooling is required, so that the effect of shortening the processing time is limited.